I have 2 9515 DD's and I am building a new box. I need 75 square inches of port area. The square port I was going to use is 5 x 15 and is suppose to be 13.75 long. Do I measure the port from the front of the wood or the back? Also how far should my port be from the back of the box? I was thinking I might have to turn the port to get the length I need. The box inside measurement is 16" so the only would give me 2.25 inches from the back of the wood and 3.75 from the front.